Rotary nickel titanium (RNT) files have been an exciting addition to the endodontic armamentarium. The files are superelastic, meaning they will regain their shape after being curved during function.
These files vary widely across different brands with regard to cross sectional design, tapers, tip sizes, helical angles, rake (cutting) angle, length, tip configuration, flute length, width and depth, presence or absence of radial lands, recommended rotational speed amongst many parameters and intended functions.
While the metal may be the same, different file brands have significant diversity comparing these designs and clinical function. RNT files of various brands and designs can create excellent prepared canal shapes easily and without significant risk of fracture if they are used appropriately. The converse is true as canal transportation and instrument fracture (amongst other potential misadventures) can result from their misuse. The above notwithstanding, these files represent a significant step forward from the traditional standard of hand filing with the use of Gates Glidden drills.
Maximising the function and performance of RNT files has numerous components, one of which is the creation of a smooth and consistent motion in their revolution within the canal as powered by an electric motor. Providing a constant torque which can be ‘auto reversed’ if a pre-set torque limit is reached is a cornerstone method of prevention of the separation of the files. A constant torque and the use of auto reverse can diminish the chances for instrument fracture.
Utilising the correct tactile sense, manual negotiation and pre-flaring of the canal with hand files to a minimal size 15 hand K file, copious irrigation, recapitulation, minimal engagement of the file flutes against the dentin wall and use of a light tactile touch during insertion are other strategies to avoid instrument separation. Lower rotational speeds are consistent with diminished instrument fracture.
Choosing an electric motor to power RNT files is not as simple as it might seem, given the wide variety of choices available on the market today. Models can be handheld without a cord, corded, and/or built into the handpiece delivery unit amongst different delivery methods.
Making decisions becomes even more complex depending on whether the clinician is building or remodelling a new office or adding electric motors into an existing office. And finally, if the clinician is trying to blend electric motors for high speed, low speed and endodontic use with the fewest number of boxes and cords, options are available.
The chosen electric motor must ideally match the best tactile and physical uses of the file. For example, using auto reverse alarms to tell the clinician when to reduce insertion speed and force is fraught with problems. While auto reverse can be a very helpful function in some clinical indications, using it in lieu of a gentle and passive file insertion will lead to the separation of files.
The mechanism of such fracture is easily described from two sources. The first is a file that becomes locked at the tip where the larger diameter of the file has enough torque to power the file; such a locked tip can easily and quickly fracture – especially if the file tip tracks a canal space that has not been first negotiated by hand and a glide path created.
Files that are deformed would also have far less resistance to torque-related failure. In addition, and more significantly, RNT files can fracture for two reasons; one is the torque related failure described above and the other is cyclic fatigue or flexural failure. A metal which is bent back and forth at the same location experiences compression on one side of the metal and tension on the other – eventually a crack will form which leads to the rupture of the metal.
A file undergoing rotation through a curvature in a canal undergoes this same set of stresses. No electric motor that the author is aware of has an auto reverse function to account for cyclic fatigue failure. New files and files of less taper are more resistant to cyclic fatigue. New files and files of greater taper are more resistant to torque failure. The converse is true for both file sizes.
Given the above considerations, two choices in electric motors stand out as excellent options given these scenarios and issues. For clinicians that have high speed handpieces and do not need new high and low speeds and just an electric motor for endodontics, the TCM III is an excellent workhorse motor (SybronEndo, Orange, CA, USA). It is simple to use, durable, easy to change settings and possesses good tactile control. For an 18:1 attachment, it has a maximum speed of 900 RPM, more than adequate for any non-surgical root canal treatment that might be indicated.
The author prefers a corded handpiece to a non-corded rechargeable one, although this is a personal preference. If the unit is attached to a power source, as the TCM III is, there is no chance that the unit will ever be unavailable because it is not charged.
Another excellent choice is the new ElectroTorque TLC by Kavo (Kavo America, Lake Zurich, IL, USA). The unit is attached to the dental unit and with one motor; two different attachments can allow either high or low speed electric handpiece function. The low speed attachment has a head for endodontics and one for low speed operatives. Both snap on to the motor, and with one push of the button on the TLC console the motor can switch between high and low speeds and immediately facilitate endodontic handpiece use for RNT files.
The TLC allows multiple presets for rotational speed and auto reverse for the torque setting if desired, which can be changed with one touch of a button. Fibre optic lights accompany the system. Obviously for someone who is happy with their handpieces, the TCM III would be an ideal option, but if one is redesigning the operatory or starting a practice, the ElectroTorque TLC is an all-in-one system for utilising RNT files in endodontics as well as all clinical operative needs.
I am not a believer in electric motors with presets for certain file brands in a given tip size and taper. Using such motors that have preset values of rational speed and auto reverse for a given torque load are more often than not somewhat arbitrary. These arbitrary values may not represent the safest or most efficient settings for these files.
In summary, rationale for using two electric motors is given with an eye towards reducing instrument fracture and improving the performance of the given files the clinician is using. I welcome your questions and feedback.